Function Definition

Specifies the index of the uniform variable to be queried. This is not a uniform location.

bufSize

Specifies the maximum number of characters OpenGL is allowed to write in the character buffer indicated by name​.

length

Returns the number of characters actually written by OpenGL in the string indicated by name​ (excluding the null terminator) if a value other than NULL is passed.

size

Returns the size of the uniform variable.

type

Returns the data type of the uniform variable.

name

Returns a null terminated string containing the name of the uniform variable.

Description

glGetActiveUniform returns information about an active uniform variable in the program object specified by program​. The number of active uniform variables can be obtained by calling glGetProgram​ with the value GL_ACTIVE_UNIFORMS. A value of 0 for index​ selects the first active uniform variable. Permissible values for index​ range from 0 to the number of active uniform variables minus 1.

Shaders may use either built-in uniform variables, user-defined uniform variables, or both. Built-in uniform variables have a prefix of "gl_" and reference existing OpenGL state or values derived from such state (e.g., gl_DepthRangeParameters​, see the OpenGL Shading Language specification for a complete list.) User-defined uniform variables have arbitrary names and obtain their values from the application through calls to glUniform​. A uniform variable (either built-in or user-defined) is considered active if it is determined during the link operation that it may be accessed during program execution. Therefore, program​ should have previously been the target of a call to glLinkProgram​, but it is not necessary for it to have been linked successfully.

The size of the character buffer required to store the longest uniform variable name in program​ can be obtained by calling glGetProgram​ with the value GL_ACTIVE_UNIFORM_MAX_LENGTH. This value should be used to allocate a buffer of sufficient size to store the returned uniform variable name. The size of this character buffer is passed in bufSize​, and a pointer to this character buffer is passed in name.​

glGetActiveUniform returns the name of the uniform variable indicated by index​, storing it in the character buffer specified by name​. The string returned will be null terminated. The actual number of characters written into this buffer is returned in length​, and this count does not include the null termination character. If the length of the returned string is not required, a value of NULL can be passed in the length​ argument.

The type​ argument will return a pointer to the uniform variable's data type. The symbolic constants returned for uniform types are shown in the table below.

Returned Symbolic Contant

Shader Uniform Type

GL_FLOAT

float

GL_FLOAT_VEC2

vec2

GL_FLOAT_VEC3

vec3

GL_FLOAT_VEC4

vec4

GL_DOUBLE

double

GL_DOUBLE_VEC2

dvec2

GL_DOUBLE_VEC3

dvec3

GL_DOUBLE_VEC4

dvec4

GL_INT

int

GL_INT_VEC2

ivec2

GL_INT_VEC3

ivec3

GL_INT_VEC4

ivec4

GL_UNSIGNED_INT

unsigned int

GL_UNSIGNED_INT_VEC2

uvec2

GL_UNSIGNED_INT_VEC3

uvec3

GL_UNSIGNED_INT_VEC4

uvec4

GL_BOOL

bool

GL_BOOL_VEC2

bvec2

GL_BOOL_VEC3

bvec3

GL_BOOL_VEC4

bvec4

GL_FLOAT_MAT2

mat2

GL_FLOAT_MAT3

mat3

GL_FLOAT_MAT4

mat4

GL_FLOAT_MAT2x3

mat2x3

GL_FLOAT_MAT2x4

mat2x4

GL_FLOAT_MAT3x2

mat3x2

GL_FLOAT_MAT3x4

mat3x4

GL_FLOAT_MAT4x2

mat4x2

GL_FLOAT_MAT4x3

mat4x3

GL_DOUBLE_MAT2

dmat2

GL_DOUBLE_MAT3

dmat3

GL_DOUBLE_MAT4

dmat4

GL_DOUBLE_MAT2x3

dmat2x3

GL_DOUBLE_MAT2x4

dmat2x4

GL_DOUBLE_MAT3x2

dmat3x2

GL_DOUBLE_MAT3x4

dmat3x4

GL_DOUBLE_MAT4x2

dmat4x2

GL_DOUBLE_MAT4x3

dmat4x3

GL_SAMPLER_1D

sampler1D

GL_SAMPLER_2D

sampler2D

GL_SAMPLER_3D

sampler3D

GL_SAMPLER_CUBE

samplerCube

GL_SAMPLER_1D_SHADOW

sampler1DShadow

GL_SAMPLER_2D_SHADOW

sampler2DShadow

GL_SAMPLER_1D_ARRAY

sampler1DArray

GL_SAMPLER_2D_ARRAY

sampler2DArray

GL_SAMPLER_1D_ARRAY_SHADOW

sampler1DArrayShadow

GL_SAMPLER_2D_ARRAY_SHADOW

sampler2DArrayShadow

GL_SAMPLER_2D_MULTISAMPLE

sampler2DMS

GL_SAMPLER_2D_MULTISAMPLE_ARRAY

sampler2DMSArray

GL_SAMPLER_CUBE_SHADOW

samplerCubeShadow

GL_SAMPLER_BUFFER

samplerBuffer

GL_SAMPLER_2D_RECT

sampler2DRect

GL_SAMPLER_2D_RECT_SHADOW

sampler2DRectShadow

GL_INT_SAMPLER_1D

isampler1D

GL_INT_SAMPLER_2D

isampler2D

GL_INT_SAMPLER_3D

isampler3D

GL_INT_SAMPLER_CUBE

isamplerCube

GL_INT_SAMPLER_1D_ARRAY

isampler1DArray

GL_INT_SAMPLER_2D_ARRAY

isampler2DArray

GL_INT_SAMPLER_2D_MULTISAMPLE

isampler2DMS

GL_INT_SAMPLER_2D_MULTISAMPLE_ARRAY

isampler2DMSArray

GL_INT_SAMPLER_BUFFER

isamplerBuffer

GL_INT_SAMPLER_2D_RECT

isampler2DRect

GL_UNSIGNED_INT_SAMPLER_1D

usampler1D

GL_UNSIGNED_INT_SAMPLER_2D

usampler2D

GL_UNSIGNED_INT_SAMPLER_3D

usampler3D

GL_UNSIGNED_INT_SAMPLER_CUBE

usamplerCube

GL_UNSIGNED_INT_SAMPLER_1D_ARRAY

usampler2DArray

GL_UNSIGNED_INT_SAMPLER_2D_ARRAY

usampler2DArray

GL_UNSIGNED_INT_SAMPLER_2D_MULTISAMPLE

usampler2DMS

GL_UNSIGNED_INT_SAMPLER_2D_MULTISAMPLE_ARRAY

usampler2DMSArray

GL_UNSIGNED_INT_SAMPLER_BUFFER

usamplerBuffer

GL_UNSIGNED_INT_SAMPLER_2D_RECT

usampler2DRect

If one or more elements of an array are active, the name of the array is returned in name​, the type is returned in type​, and the size​ parameter returns the highest array element index used, plus one, as determined by the compiler and/or linker. Only one active uniform variable will be reported for a uniform array.

Uniform variables that are declared as structures or arrays of structures will not be returned directly by this function. Instead, each of these uniform variables will be reduced to its fundamental components containing the "." and "[]" operators such that each of the names is valid as an argument to glGetUniformLocation​. Each of these reduced uniform variables is counted as one active uniform variable and is assigned an index. A valid name cannot be a structure, an array of structures, or a subcomponent of a vector or matrix.

The size of the uniform variable will be returned in size​. Uniform variables other than arrays will have a size of 1. Structures and arrays of structures will be reduced as described earlier, such that each of the names returned will be a data type in the earlier list. If this reduction results in an array, the size returned will be as described for uniform arrays; otherwise, the size returned will be 1.

The list of active uniform variables may include both built-in uniform variables (which begin with the prefix "gl_") as well as user-defined uniform variable names.

This function will return as much information as it can about the specified active uniform variable. If no information is available, length​ will be 0, and name​ will be an empty string. This situation could occur if this function is called after a link operation that failed. If an error occurs, the return values length​, size​, type​, and name​ will be unmodified.

Notes

The double types, GL_DOUBLE, GL_DOUBLE_VEC2, GL_DOUBLE_VEC3, GL_DOUBLE_VEC4, GL_DOUBLE_MAT2, GL_DOUBLE_MAT3, GL_DOUBLE_MAT4, GL_DOUBLE_MAT2x3, GL_DOUBLE_MAT2x4, GL_DOUBLE_MAT3x2, GL_DOUBLE_MAT3x4, GL_DOUBLE_MAT4x2, and GL_DOUBLE_MAT4x3 are only available if the GL version is 4.1 or higher.